State of the art x-ray tube rotating target anodes can be categorized as one of two types; anode grounded and cathode grounded. Anode grounded x-ray tubes are more expensive to produce because of a complicated stainless steel vacuum envelope and cathode structure. Furthermore, the power supplies that are required to operate such tubes are more complicated and expensive because the filament and some control circuitry are at a high potential (up to 50 kilovolts). However, the anode grounded x-ray tubes generally provide clearer images as compared to cathode grounded x-ray tubes. This is because the anode grounded structure and the associated steel vacuum envelope collect many of the scattered electrons that would otherwise strike the target and generate undesirable off-focus radiation. However, the uncollected scattered electrons which manage to strike the target anode produce off-focus x-rays. These off-focus x-rays reduce image clarity by increasing the background film blackening without producing an image.
Cathode grounded x-ray tubes are less expensive to produce because of simpler vacuum envelopes and cathode structures. In addition, the power supplies are considerably less expensive because the filament and all control circuitry can be operated at or near ground potential. The trade-off comes in performance. The cathode grounded x-ray tube generally suffers several times the amount of off-focus or extra-focal radiation because most scattered electrons return to the target anode and produce undesirable x-rays. The resulting x-ray images are more fogged than images from anode grounded x-ray tubes. Despite these drawbacks, they are often ignored in light of the substantial cost savings of the cathode grounded x-ray tube design.
It would be advantageous to be able to provide a cathode grounded x-ray tube which could produce x-ray images on screen or film which are comparable to the clarity of images from anode grounded x-ray tubes.
The present invention is designed to overcome the problems presented by off-focus x-ray radiation. Off-focus radiation is also referred to as extra-focal radiation. Focal radiation is radiation that carries information to the film or screen making the x-ray image. Extra-focal radiation is produced by electrons which are back scattered from the target focal spot and land on the x-ray tube target. But as is understood, a significant source of extra-focal radiation is produced by electrons which strike a rotating target anode in areas other than a focal spot toward which electrons are directed. Any radiation generated outside of the focus spot can only degrade the diagnostic image generated on the film. It is noted that film also refers to any other x-ray sensitive image device or surface.
FIG. 1 shows a target anode 20 which is ready to be disposed within an x-ray tube assembly. The target anode 20 generates extra-focal radiation from electrons 8 which "rebound" from near a focal spot 24 and fall back to the target anode 20 outside of a focal area 23. Wherever these electrons 8 land on the target anode 20, they produce x-rays. Because these x-rays are almost as penetrating as those from the focal spot 24, the deliberate addition of filtration between the target anode 20 and an x-ray sensitive film will generally not significantly improve matters. These electrons 8 can even curve all the way around to a back face of the target anode 20, and then generate off-focus radiation.
The problem of off-focus radiation is not insignificant in its severity. FIG. 2 is provided as an image of extra-focal radiation as seen by a pin-hole camera. The dark spot 6 in the middle of the image is radiation from the focal spot 24 (FIG. 1). However, not only an outline but an entire image of the target anode 20 is clearly visible in profile. Thus, there is substantial off-focus radiation being generated which reduces resolution, clarity and contrast of resulting x-ray images. Furthermore, the effect is much more pronounced for rotating target anodes than for stationary anodes because of the greater area of tungsten or tungsten carbide generally involved. In the stationary target anode tube design, much of the rebounding electron shower falls on copper, thus resulting in a lower level of extra-focal radiation.
FIG. 3 is provided as a more detailed illustration of a typical prior art cathode grounded x-ray tube assembly 10 which shows the features which are most relevant to the present invention. There are other tilt angles and designs. Nevertheless, depending upon the mammography system design, the x-ray housing 12 of the cathode grounded x-ray tube assembly 10 is usually tilted at an angle 15 of about six degrees with respect to a horizontal plane 14. The exact tilt of the x-ray housing 12 and a resulting target angle 16 of the target anode 20 is determined by an x-ray source-to-film distance 18. In this figure, the target angle 16 is chosen to be at a sixteen degree angle relative to a common or central axis 39 of the target anode 20 and the x-ray housing 12.
FIG. 3 also shows the possible paths that x-rays can travel from the target anode 20 to an x-ray sensitive imaging device 22. In this case, the x-ray sensitive imaging device 22 is shown as a portion of an x-ray sensitive imaging device. Ideally, the only radiation from the target anode 20 that strikes the x-ray sensitive imaging device 22 would be x-rays generated at the focal spot 24. The width of a path of x-rays generated from the focal spot 24 is delineated by solid lines 26. Therefore, the portion of the x-ray imaging sensitive device 22 which is of concern to the present invention falls between the path represented by the solid lines 26. The extent of coverage of the x-ray sensitive imaging device 22 is thus shown as width 28.
If the focal spot 24 was the only source of radiation which would impinge upon the x-ray sensitive imaging device 22, there would be less problems with the quality of x-ray images generated thereby. However, FIG. 1 also shows other surfaces of the target anode 20 which function as undesirable sources of radiation (off-focus radiation) which can strike the x-ray sensitive imaging device 22. Specifically, a back face 30, an edge face 32, a target face 33 and even the front face 34 of the target anode 20 are all off-focus radiation sources. It should also be mentioned that portions of a focal spot track 36 (not seen in this profile view of the target anode 20), which at any given moment are not the focal spot 24, can also be a source of off-focus radiation.
Having identified the off-focus radiation producing surfaces 30, 32, 33 and 34 of the target anode 20, it is now useful to see the possible paths that the radiation can follow from all these surfaces to the x-ray sensitive imaging device 22. A possible path of off-focus radiation from the back face 30 is shown as dotted lines 36. Possible paths of off-focus radiation from the edge face 32 are shown as dotted lines 38 (a path also partially shared with dotted line 36). Finally, a possible path of off-focus radiation from the target face 34 is shown as dotted line 40. All of these paths 36, 38, and 40 are possible routes for x-rays to travel from the target anode 20 to the x-ray sensitive imaging device 22. What is important to observe is that all the paths fall within the desired extent of coverage 28 of radiation from the focal spot 24. This desired extent of coverage 28 can also be referred to as a focal direction.
Therefore, what is needed is a way to reduce off-focus or extra-focal radiation when using a rotating target anode in a cathode grounded x-ray tube. It would be a further advantage to increase the clarity of x-ray images generated by such an x-ray tube as compared to anode grounded x-ray tubes.
One method for reducing off-focus radiation used in the prior art is to dispose various apertures between the target anode and an x-ray sensitive imaging system. Strategic placement of apertures attempts to limit radiation which is coming from off-focus radiation sources on the target anode. Unfortunately, for various reasons this method also causes other problems, making it only partially effective for off-focus radiation reduction. Therefore, what is also needed is a way to decrease off-focus radiation at the source of most x-ray emissions, the target anode.
It is an object of the present invention to provide a method and apparatus for reducing off-focus radiation of a target anode in a cathode grounded x-ray tube.
It is another object to provide a method and apparatus for achieving higher resolution in x-ray images generated from the present invention as compared to state of the art cathode grounded x-ray tubes.
It is another object to provide a method and apparatus for achieving greater clarity in x-ray images generated from the present invention as compared to state of the art cathode grounded x-ray tubes.
It is another object to provide a method and apparatus for achieving lower density contrast in x-ray images generated from the present invention as compared to state of the art cathode grounded x-ray tubes.
It is another object to provide a method and apparatus for reducing off-focus radiation from a target anode used in a cathode grounded x-ray tube by modifying a shape of the target anode.
It is another object to provide a method and apparatus for reducing off-focus radiation from a target anode used in a cathode grounded x-ray tube by modifying angles of the target anode so that scattered electrons which generate off-focus radiation from the target anode are not directed towards an x-ray sensitive imaging device.
It is another object to prove a method and apparatus for reducing off-focus radiation from a target anode used in a cathode grounded x-ray tube such that x-ray images generated therefrom are comparable to x-ray images generated by anode grounded x-ray tubes.
It is another object to provide a method and apparatus for reducing off-focus radiation from in a cathode grounded x-ray tube by reducing the off-focus radiation at the source.
It is another object to provide a method and apparatus for reducing off-focus radiation from a target anode used in a cathode grounded x-ray tube by modifying widths of surfaces which can generate the off-focus radiation